Two men serving lengthy prison sentences for murder and attempted murder are accused in the Monday murder of an inmate at the Sterling Correctional Facility, authorities say.

Robert Sprowls, serving a 48-year sentence for attempted murder out of Arapahoe County, and Thomas Johns, serving a life sentence for murder out of Jefferson County, are suspects in the death of Cody Gray, said Adrienne Jacobson, spokeswoman for the Colorado Department of Corrections.

Gray was serving a life sentence out of Mesa County for crimes including raping a jail inmate, according to media reports. Gray is the sixth inmate murdered at the prison since 2010.

Colorado prison chief Rick Raemisch has ordered a review of the circumstances leading to the six murders.

Robert Sprowls (Provided by Colorado Department of Corrections)

"It's a high number," Jacobson said, referring to the six deaths. "We'll look at whether there is an issue at Sterling or if it is related to the prison's size."

Sprowls, Johns and Gray were serving in a high-security, "closed custody" cellblock at Sterling. Gray's body was found Monday morning in a cell.

The review ordered by Raemisch will consider issues including the classification of inmates, which dictates whether they are placed in high-, medium- or minimum-security prisons.

Gray, Sprowls and Johns were at the second-highest security level in the prison system. Only maximum-security offenders in 23-hour lockdown are in a higher level of security.

Johns, 24, was 17 on July 15, 2007, when he and an accomplice robbed and fatally shot 30-year-old Celia Meza at a Burger King at West 64th Avenue and Ward Road. Before the murder, Johns and Anthony Lowe joked about throwing an employee in the freezer. That is where Lowe fatally shot Meza as she called 911 on her cellular phone. Johns, who previously worked at the restaurant, was described as the mastermind.

Sprowls, 35, fired a gun at an Aurora police officer on Nov. 30, 2004, during a foot chase, according to an arrest affidavit.

Arizona prison officials had "strongly recommended" that Farley not be held with a cellmate because of the "risk he would harm that person." But instead, prison officials placed Farley directly into a general population cellblock at Sterling.

Farley had sexually assaulted a cellmate after threatening him with a razor blade in 1999. He sliced a second cellmate down the spine with a razor blade in 2000, and while in segregation, he fashioned a noose out of a braided bed sheet, passed the noose to a neighboring inmate who was suicidal and strangled him to death.